FIGS. 1 to 3 show the etching steps of a conventional method for preparing a stacked capacitor. As shown in FIG. 1, a substrate 12 is provided. The substrate 12 includes four gate structures 14, a bit-line contact plug 16, two capacitor contact plugs 18, 18a and a dielectric layer 20. A carbon hard mask 10 is applied on the substrate 12. The carbon hard mask 10 has a pattern 101 to expose part of the substrate 12.
As shown in FIG. 2, the substrate 12 with the carbon hard mask 10 is placed in a plasma etching apparatus 2. The plasma etching apparatus 2 comprises a container 21, an upper electrode plate 23, a lower electrode plate 25, a gas source 26, a gas exhaust unit 27, an upper source RF power supply 28, an upper source RF power supply controller 281, a DC power supply 29, a DC power supply controller 291, a bias RF power supply 30, a bias RF power supply controller 301, a lower source RF power supply 31 and a lower source RF power supply controller 311.
The container 21 defines a processing chamber 22 and is electrically grounded. The upper electrode plate 23 is disposed in the processing chamber 22. The lower electrode plate 25 is disposed in the processing chamber 22 and includes a chuck 24 for holding the substrate 12. The gas source 26 is connected to the processing chamber 22 for introducing a processing gas into the processing chamber 22. Usually, the gas source 26 comprises an etch gas source 261, a deposition gas source 262 and a gas controller 263. The etch gas source 261 supplies an etch gas to the processing chamber 22 and the deposition gas source 262 supplies a deposition gas to the processing chamber 22 through the gas controller 263. The gas exhaust unit 27 is used for removing the gas from the processing chamber 22 so as to control the pressure in the processing chamber 22.
The upper source RF power supply 28 is controlled by the upper source RF power supply controller 281, and is electrically connected to the upper electrode plate 23 for continuously supplying an upper source RF power to the upper electrode plate 23 during a plasma etching process. The DC power supply 29 is controlled by the DC power supply controller 291, and is electrically connected to the upper electrode plate 23 for continuously supplying a DC power to the upper electrode plate 23 during the plasma etching process.
The bias RF power supply 30 is controlled by the bias RF power supply controller 301, and is electrically connected to the lower electrode plate 25 for continuously supplying a bias RF power to the lower electrode plate 25 so as to generate a plasma in the processing chamber 22 to etch the substrate 12. The lower source RF power supply 31 is controlled by the lower source RF power supply controller 311, and is electrically connected to the lower electrode plate 25 for continuously supplying a lower source RF power to the lower electrode plate 25.
As shown in FIG. 3, during the etching process, the dielectric layer 20 of the substrate 12 is etched to form two high-aspect-ratio trenches 19, 19a. The trench 19 exposes the capacitor contact plug 18 and is considered as a qualified trench. The trench 19a has a twisted profile that does not expose the capacitor contact plug 18a, and thus is considered a non-qualified trench. In the subsequent process, the conductive layer deposited in the trench 19a cannot connect to the capacitor contact plug 18a, making an open circuit. Alternatively, the trench 19a may expose another capacitor contact plug, allowing the conductive layer deposited in the trench 19a to connect to the unexpected capacitor contact plug, making a short circuit.
The formation of the trench 19a is described as follows. During the etching process, most of the electrons 21b are distributed around the carbon hard mask 10, and a large amount of the positive ions 21a penetrate deeply into the trench 19a. Because there are too many positive ions 21a on the bottom of the trench 19a, the trajectories of the following positive ions are bent, causing the twisting profile of the trench 19a. In addition, the unbalanced concentration between the etch gas and the deposition gas also influences the profile of the trench 19a. 
In order to improve the above-mentioned problem, the DC power supply 29 is used to continuously supply a DC power to the upper electrode plate 23 to induce the secondary electron emission. The secondary electrons are expected to pass through the bulk plasma and sheath, then enter the trench 19a to neutralize the positive ions 21a. However, in fact, the secondary electrons need super high energy to pass through the bulk plasma and sheath, and less than 6% of the secondary electrons are able to reach the substrate 12. Thus, the DC power superposition is not able to eliminate the twisting profile of the trench 19a. 